1. Field of the Invention
The present invention relates to an oxide semiconductor electrode for a dye sensitized solar cell and a process for producing the same.
2. Description of the Related Art
Solar cells utilizing sunlight have drawn attention as an energy source that is cleaner than energy sources which rely upon fossil fuels. Among others, solar cells utilizing pn junction of semiconductor silicon are well known and have been put to practical use. Silicon solar cells, however, require large energy in the production thereof, particularly in the purification of silicon as a raw material. Therefore, silicon solar cells involve problems including that a reduction in price is difficult and the power generation cost is high.
On the other hand, constructing an electrochemical solar cell using a dye has been proposed in Japanese Unexamined Patent Publication (Kokai) No. 1-220380. Solar cells of this type are generally called “dye sensitized solar cells,” and the construction thereof is shown in FIG. 1. As shown in FIG. 1, a dye sensitized solar cell 1 comprises an oxide semiconductor electrode 2, a counter electrode 3 provided opposite to the oxide semiconductor electrode 2, and an electrolyte layer 4 interposed between the oxide semiconductor electrode 2 and the counter electrode 3. The oxide semiconductor electrode 2 comprises a glass substrate 5, a transparent conductive layer 6 provided on the glass substrate 5, and an oxide semiconductor layer 7 provided on the transparent conductive layer 6. On the other hand, the counter electrode 3 comprises a glass substrate 8 and a transparent conductive layer 9 provided on the glass substrate 8. The electrolyte 4 comprises couples of iodine ions (I−/I3−) with a plurality of different oxidation states.
FIG. 2 is a partially enlarged view of the dye sensitized solar cell 1 shown in FIG. 1. As shown in FIG. 2, the oxide semiconductor layer 7 comprises a plurality of titania (TiO2) particles 10 which have been joined to each other. A dye 11 of a ruthenium complex is chemically adsorbed on the titania particles 10.
Upon the entry of light through the glass substrate 8, at the boundary of the titania particles 10 in the oxide semiconductor layer 7, the dye 11, and the electrolyte layer 4, three iodide ions (I−) in the electrolyte give off two electrons and consequently are oxidized to iodide ions (I3−). That is, the electrolyte functions as an oxidation-reduction agent. Due to a potential gradient, the electrons arrive at the transparent conductive layer 6 and further reach the counter electrode 3. On the other hand, iodide ions (I3−), which have been brought to a higher oxidation state, travel through the electrolyte layer 4 and arrive at the transparent conductive layer 9 in the counter electrode 3. Here the iodide ions (I3−) receive two electrons and consequently are reduced to iodide ions (I−) in a lower oxidation state, which again travel to the boundary of the titania particles 10, the dye 11, and the electrolyte layer 4. The repetition of the above process causes solar energy to be converted to electric energy, and a current flows.
Thus, in the dye sensitized solar cell, as electrons are smoothly given and received through an electrolyte, the oxide semiconductor layer is generally made porous. The oxide semiconductor layer has hitherto been formed by providing a glass substrate with a transparent conductive layer formed thereon, coating a slurry or paste of a fine powder of an oxide semiconductor, such as titania, onto the transparent conductive layer provided on the glass substrate, and baking the coating. In this case, however, as the baking temperature is high and is 400 to 700° C., this method cannot be applied to a resin or other substrate and glass is used as the substrate. Therefore, although the realization of a flexible solar cell has been advocated from theoretical viewpoint, a flexible solar cell has not been realized yet.
As described above, the oxide semiconductor layer is porous. In order to increase dye adsorption, the oxide semiconductor layer is formed of oxide semiconductor particles having a very small diameter. Therefore, light, in the visible to near-infrared regions involved in photoelectric conversion, is disadvantageously transmitted without satisfactory absorption in the oxide semiconductor layer, and satisfactory power generation efficiency cannot be provided.
Further, voids within the oxide semiconductor layer are so small that, in the conventional method wherein the oxide semiconductor layer is dipped in a dye solution, much time is necessary for the adsorption of the dye and, further, it is difficult to diffuse and adsorb the dye in a region within the oxide semiconductor layer.